1. FIELD OF THE INVENTION
This invention relates to a calendering apparatus for hot-calendering a paper web. More particularly, this invention relates to supercalendering and gloss calendering in which a web of paper extends through a calendering nip defined by a hard metal roll and a soft calender roll.
2. DESCRIPTION OF THE RELATED ART
In order to improve the finish of a dried web, the web of paper is pressed, or calendered, between a pair of cooperating rolls. Such calendering creates a smoother surface on the web and reduces the caliper, or thickness, of the web. Additionally, such calendering thermally bonds the fibers within the web. The aforementioned devices which include machine calenders, supercalenders and gloss calenders are utilized in the papermaking art to achieve the aforementioned required properties.
Research studies and practical experience have taught that the finishing operation on a paper web can be enhanced by heating at least one of the nipped rolls. Such heated roll then heats the web as it passes through the calendering nip and thereby effects the calendering action.
In order to provide the high roll surface temperatures required to enhance the calendering process, such rolls have been heated by hot water which is pumped through channels extending through the heatable roll. However, in a typical calendering roll, the roll shell is normally very thick in order to support the mechanical load of the nip. As a result of such thick shell, the water circulating through the calendering roll is unable to supply heat to the surface of the roll at a fast enough rate to maintain a high surface temperature. The aforementioned heating systems have generally only been successful when controlling the temperature at the edges of the roll where the heat flow is much smaller.
In an attempt to improve the rate of heat transfer to the roll surface, proposals have been made wherein the internal area of the roll has been fitted with a "displacer shell". "Displacer shells" act to reduce the cross sectional area for water flow and thereby increase the velocity of the water and produce a higher convective heat transfer coefficient. Nevertheless, the thick shell of the calendering roll still produces a significant resistance to heat transfer so that the gains made by the "displacer shell" are usually small. In the prior art, proposals have been set forth in an effort to further increase the temperature of the calendering roll by the utilization of oil in place of heated water. Such heating oil is sold under the Registered Trademark MOBILTHERM. MOBILTHERM is a Registered Trademark owned by Mobil Oil Corporation. Such heat transfer fluid can be heated to a temperature of between 500.degree. to 550.degree. Fahrenheit without the correspondingly high vapor pressure of water. Although these alternate heat transfer fluids create higher surface temperatures, they are still limited by the thick shell of the calendering roll in terms of temperature range and they also result in high shell thermal stresses.
In the prior art, another alternative has been proposed in which a series of axial holes are milled in the shell of the calendering roll. The shell is then covered with an outer surface sleeve so that the heat transfer fluid can circulate in the vicinity of the calendering surface. Those portions, or lands, between adjacent holes provide the mechanical support of the nip load so that the distance between the heated fluid and the roll surface can be greatly reduced. Alternatively, the surface of the shell is radially drilled with a plurality of holes. One drawback of the aforementioned proposals is that the heating of the surface of the calendering roll is not completely uniform in a circumferential direction. Additionally, there exist large thermal gradients in the shell which cause thermal stresses.
In addition to the aforementioned problems, another problem presents itself when operating a calender at elevated temperatures in that there exists a non-uniformity in the surface temperature in a cross-machine direction. Such surface temperature is influenced not only by variations in the web weight but also by moisture content and nip pressure. Also, the temperature of the web has a bearing on the cross-machine direction profile.
In the prior art, several proposals have been made utilizing external induction heating devices to supply the required thermal energy to the calendering roll. One example of an induction heated-calender is European patent application No. 159337 to Valmet. European patent application number 159337 recognizes the problem of controlling the cross-machine direction heating of the calendering roll. The solution of this problem, according to EP No. 159337, involves mounting each segment, or shoe, of an induction heating means so that each shoe can be selectively pivoted towards, or away from, the surface of the calendering roll thereby controlling the temperature of that portion of the roll adjacent to a particular shoe.
The aforementioned solution to the control of cross-machine direction temperature requires relatively complex mounting of each individual induction heater.
The present invention seeks to overcome the aforementioned problem by selectively directing a plurality of curtains of cooling air against the calendering surface of the calendering roll so that the cross-machine direction temperature adjacent to the calendering nip is controlled.
Therefore, it is a primary object of the present invention to provide a calendering apparatus that overcomes the aforementioned problems associated with the prior art hot-calendering arrangements and which provides a significant contribution to the paper calendering art.
Another object of the present invention is the provision of a calendering apparatus in which an air blowing means is disposed in close proximity to the calendering surface and between the heating means and the calendering nip with the blowing means defining a plurality of compartments disposed in a cross-machine direction. Each of the compartments selectively blows a current of cooling air against the calendering surface for controlling the temperature of the calendering surface so that the temperature of the calendering surface adjacent to the calendering nip is controlled in a cross-machine direction.
Another object of the present invention is the provision of a calendering apparatus having the aforementioned air blowing means and wherein the calendering roll includes a core, an annular insulating layer surrounding the core and a heatable annular shell surrounding the insulating layer such that eddy currents generated by means of an induction heater are localized in the heatable annular shell.
Another object of the present invention is the provision of an air gap between the core of the induction heater and the calendering surface, the air gap progressively increasing towards the side edges of the core in order to inhibit overheating of the lateral edges of the web.
Other objects and advantages of the present invention will be apparent to those skilled in the art by a consideration of the detailed description contained hereinafter taken in conjunction with the annexed drawings.